U.S. Pat. No. 4,712,906, Bothner et al., shows an electrophotographic color printer which forms consecutive images in different colors that are transferred in registry to a receiving sheet. The receiving sheet is wrapped on a transfer drum or roller and recirculated on the surface of the drum into transfer relation with the consecutive images to create a multicolor image on the sheets. To improve efficiency, large sheets, for example, "ledger" size sheets are placed on the drum with the small dimension parallel to the axis of the drum and wrapped substantially around the transfer drum. Small sheets, for example, "letter" size sheets are placed with their long dimension parallel to the axis of the drum. Since the short dimension of letter size sheets is approximately half the long dimension of ledger size sheets, two letter size sheets are placed on the drum in approximately the same space as the single ledger size sheet.
Prior to the Bothner invention, commercial color image transfer devices secured the receiving sheet to the transfer drum with small gripping fingers that grip the leading edge of the sheet. Many other methods are mentioned in the literature, for example, vacuum holes, electrostatics or various combinations of vacuum holes, electrostatics and gripping fingers. The gripping fingers were preferred commercially because they more firmly hold the sheet against slippage, which slippage would degrade the registration of the color images.
However, the Bothner invention is difficult to utilize with gripping fingers because the leading edge of the second letter size sheet is positioned at approximately the middle of a ledger size sheet. For some applications, retractable fingers may be made to work, but for many applications they would leave substantial image artifacts in a ledger size sheet. Bothner therefore suggests the use of vacuum holes which are positioned at the leading edge of each of the smaller sheets and may or may not both be activated for the ledger size sheet.
To firmly hold fairly heavy stock the holes were made as large as 3.6mm in diameter and placed less than one to a centimeter in a line across the drum.
The vacuum holes shown in Bothner work fine in many situations. However, under some conditions, the vacuum holes show up on the final image as small round areas of incomplete toner transfer. This is especially true in dry ambient conditions, with transparency receiving stock and with the second transfer to duplex receiving sheets where the receiving sheet has been dryed by a prior fusing step.
Even in dry conditions, the artifacts may be acceptable if they were confined to the leading edge of all sheets where image information is unlikely. However, the Bothner apparatus forces at least one line of vacuum holes for the leading edge of the second small sheet, to the middle of a large sheet. Further, in different sheet holding applications, it may be necessary to put vacuum holes at the trailing edge as well as the leading edge of at least some sheets. If a variety of sheet sizes is to be available, many lines of trailing edge holes will be necessary. Vacuum holes on the trailing edges of a variety of sheets place many lines of holes in the middle of larger sheets, depending on the mixture of sizes available in the machine.
U.S. Pat. No. 4,080,053, Friday, shows a vacuum web transport for a copy sheet through a transfer station having a rather lengthly transfer area formed by parallel portions of the transfer web and a photoconductive web. To prevent what the reference termed "vacuum hole printout", the effective position of the holes is gradually moved to different locations during passage through the transfer zone. Whatever the effectiveness of this solution for the apparatus shown, it would not be useful with the relatively small transfer zone formed by a transfer drum with either an image carrying web or drum.
The Bothner apparatus shows a transfer drum having an aluminum base with a polyrethane coating of intermediate conductivity. The layer of intermediate conductivity allows the creation of a relatively strong transfer electric field without electrical breakdown in the nip. It is believed that the failure to transfer toner over a vacuum hole is due to lack of continuity of the electric field in that region when a less conductive, for example, a dryed out transfer sheet is being used.